This invention relates to an ink jet recording apparatus comprising a recording head for jetting ink drops through nozzle orifices by selectively supplying a drive signal to each piezoelectric vibrator.
An ink jet recording apparatus such as an ink jet printer comprises a recording head having a large number of nozzle orifices formed as the nozzle orifices are arranged like rows. The recording head may comprise pressure generating chambers communicating with the nozzle orifices and piezoelectric vibrators for expanding and contacting the pressure generating chambers, thereby changing the ink pressure in each pressure generating chamber. In such a recording head, a drive signal is supplied to each piezoelectric vibrator, thereby changing ink pressure for jetting an ink drop through the nozzle orifice.
For an ink jet recording apparatus of this kind, an art for jetting different amounts of ink drops through one nozzle orifice is proposed to satisfy contradictory needs for improving the image quality and improving the record speed. For example, a necessary waveform element is selected appropriately from a drive signal sequence comprising a plurality of waveform elements connected and the selected waveform element is supplied to a piezoelectric vibrator, thereby jetting a plurality of types of ink drops different in amount.
Each piezoelectric vibrator used with the recording head is considered as an ideal condenser. That is, it is considered that the potential of the piezoelectric vibrator (segment potential) continues to hold the potential at the point in time at which the previous waveform element supply is stopped. Based on this consideration, the termination potential of the previously supplied waveform element and the initial potential of the later supplied waveform element are put into the same level.
However, it is found that the actual piezoelectric vibrator has insulating resistance and that if it is left standing without supplying a drive signal, the potential gradually drops because of natural discharge. The potential drop is caused by non uniformity of piezoelectric substance in piezoelectric layer, for example.
The longer the time between the supply end of the previous waveform element and the supply start of the subsequent waveform element, namely, the longer the supply period of no drive signal, the larger the potential drop amount.
The potential drop is noticeable as the film thickness is thinner (the field effect strength becomes stronger). Thus, with the recently required piezoelectric vibrator having a small volume (thin thickness), the difference between the potential and the initial potential of the subsequent waveform element becomes large if the supply period of no drive signal is extremely short.
If the subsequent waveform element is supplied to the piezoelectric element in a state in which the difference between the potential and the initial potential of the subsequent waveform element is large, the piezoelectric potential varies largely in a short time just after supply of the waveform element starts. Consequently, rapid deformation occurs in the piezoelectric vibrator and an ink drop is jetted in error; this is a problem. Since a large current exceeding the allowable range flows into the piezoelectric vibrator, the function of the piezoelectric vibrator can be degraded and the piezoelectric vibrator can be destroyed.
It is therefore an object of the invention to prevent ink drops from being jetted in error and a piezoelectric vibrator from being destroyed for enhancing the reliability of a recording apparatus by easing a rapid rise in the potential in the piezoelectric vibrator.
In order to achive the above object, according to the present invention, there is provided an ink jet recording apparatus comprising:
a recording head including a piezoelectric vibrator to be deformed for varying the volume of a pressure generating chamber communicated with a nozzle orifice to eject an ink drop therefrom;
a drive signal generator for generating a drive signal in which a plurality of waveform elements are connected;
a waveform element supplier for selectively supplying at least two waveform elements from the drive signal to the piezoelectric vibrator; and
a potential difference compensator for compensating a potential difference between a termination potential of a previous waveform element and an initial potential of a subsequent waveform element respectively selected by the waveform element supplier.
According to the above configuration, the termination potential of the previous waveform element and the initial potential of the subsequent waveform element can be matched with each other by the potential difference compensator. Thus, a rapid rise in the potential of the piezoelectric vibrator when the subsequent waveform element is supplied can be eased. Therefore, the disadvantages involved in the rapid rise in the potential, such as erroneous jetting of an ink drop and destroying of the piezoelectric vibrator, can be prevented. Consequently, the reliability of the recording apparatus can be enhanced.
Prefereably, the potential difference compensator includes:
a first supply switch connected between the piezoelectric vibrator and a drive signal line for supplying the drive signal to the piezoelectric vibrator in order to control the waveform element supply;
a second supply switch connected between the drive signal line and the piezoelectric vibrator in parallel with the first supply switch;
a first switch controller for supplying a first switching signal to the first switch for controlling the same;
a second switch controller for supplying a second switching signal to the second switch for controlling the same; and
a rectifier connected in serial with the second supply switch such that a current flow direction from the drive signal line to the piezoelectric vibrator is defined as a forward direction.
Preferably, the drive signal includes a first waveform element for deforming the piezoelectric vibrator and, a second waveform element, one portion of which is convex to the lower potential side;
wherein the first switch controller generates the first switching signal in at least a part of a period while the first waveform element is generated; and
wherein the second switch controller generates the second switching signal at a predetermined timing in a period while the second waveform element is generated so as to turn on the second supply switch.
Preferably, the lowest potential of the second waveform element is lower than a potential of the piezoelectric vibrator at the time where the second switch is turned on.
Preferably, the second switch controller generates the second switching signal for turning on the second switch in a period while the potential of the second waveform element is rising.
According to the above configuration, an electric current flows into the piezoelectric vibrator through the second supply switch and the rectifier only when the waveform element potential becomes higher than the piezoelectric potential by previously turning on the second supply switch in a state in which the potential lowers. Consequently, a rapid rise in the potential can be eased.
Alternatively, the potential difference compensator includes an initial potential setter for lowering an initial potential of the subsequent waveform element than a termination potential of the previous waveform element in response to potential drop of the piezoelectric vibrator.
Preferably, the subsequent waveform element includes a compensation element for restoring the potential thereof from the initial potential to the termination potential of the previous waveform element.
Preferably, the waveform supplier includes a compensation element collective supplier for supplying the compensation element to every piezoelectric vibrator collectively.
Preferably, the waveform supplier includes a compensation element selectively supplier for supplying the compensation element to a predetermined piezoelectric vibrator selectively.
Preferably, the initial potential setter includes an initial potential adjuster for adjusting the initial potential of the subsequent waveform element.
Preferably, the ink jet recording apparatus further comprises an environment information detector for acquiring information of an environment surrounding the recording head. The environment information includes at least one of temperature information and humidity information. The initial potential adjuster adjusts the initial potential of the subsequent waveform element in accordance with the detected environment information.
Preferably, the initial potential adjuster adjusts the initial potential of the subsequent waveform element such that the difference between the termination potential of the previous waveform element and the initial potential of the subsequent waveform element becomes larger as the termination potential of the previous waveform element is higher.
Preferably, the initial potential adjuster adjusts the initial potential of the subsequent waveform element such that the difference between the termination potential of the previous waveform element and the initial potential of the subsequent waveform element becomes larger as the time period between the termination of the previous waveform element and the start end of the subsequent waveform element is longer.
Preferably, the ink jet recording apparatus further comprises a potential detector for detecting a potential of the piezoelectric vibrator. The initial potential adjuster adjusts the initial potential of the subsequent waveform element in accordance with the detected potential of the piezoelectric vibrator.
According to the above configuration, the difference between the potential and the initial potential of the waveform element can be made extremely small even if the potential is dropped during the period in which no drive signal is supplied, namely, from the time at which supply of the previous waveform element is terminated to the time at which supply of the subsequent waveform element is started. Thus, a rapid rise in the potential just after supply of the subsequent waveform element is started can be prevented.
According to the present invention, there is also privided an ink jet recording head comprising:
a recording head including a piezoelectric vibrator to be deformed for varying the volume of a pressure generating chamber communicated with a nozzle orifice to eject an ink drop therefrom;
a drive signal generator for generating a drive signal in which a plurality of waveform elements are connected;
a waveform element supplier for selectively supplying at least one waveform element from the drive signal to the piezoelectric vibrator; and
a termination potential supplier for maintaining a potential of the piezoelectric vibrator during a period in which no waveform element is supplied at a termination potential of a reference waveform element supplied just before.
Preferably, the termination potential supplier supplies a potential corresponding to the termination potential of the reference waveform element after the waveform supplier has been finished to supply the reference waveform element.
Preferably, the waveform element supplier includes a first selective switch for selectively supplying at least one of the waveform elements in the drive signal to the piezoelectric vibrator. The termination potential supplier includes a second selective switch for supplying the potential corresponding to the termination potential of the reference waveform element to the piezoelectric element selectively.
Preferably, the ink jet recording apparatus further comprises a print controller for generating a latch signal. The first selective switch is on/off controlled by the latch signal and the second selective switch is controlled on/off controlled by a signal generated by inverting the latch signal.
Preferably, the second selective switch is turned on after the expiration of a predetermined time period since the first selective switch has been turned off.
Preferably, the predetermined time period is set in the range of 5 xcexcsec to 20 xcexcsec.
Preferably, the termination potential supplier includes a supply source for supplying the termination potential and a current limiter provided between the supply source and the second selective switch.
Preferably, the current limiter is configured as a resistance element connected in series between the supply source and the second selective switch.
Preferably, the waveform element supplier includes a first selective switch for selectively supplying at least one of the waveform elements in the drive signal to the piezoelectric vibrator. The termination potential supplier includes a second selective switch for selectively supplying one of predetermined different potentials in response to a termination potential of a waveform element selectively supplied by the first selective switch.
According to the above configuration, the potential drop caused by natural discharge occurring while the period in which no waveform element is supplied can be suppressed. Therefore, rapid vibration in the potential just after supply of the next drive signal is started can be prevented and the disadvantages involved in the rapid variation in the potential, such as erroneous jetting of an ink drop and degradation or destroying of the piezoelectric vibrator, can be prevented.
Preferably, the piezoelectric vibrator is configured as a piezoelectric element.
Preferably, the piezoelectric vibrator includes a piezoelectric layer and electrodes disposed so as to sandwich the piezoelectric layer. The piezoelectric layer has a thickness in the range of 1 xcexcm to 20 xcexcm.